Reference to background documents refers to the “List of References” provided below.
The demand on optical devices and technologies in the fields of signal processing, mobile communication technologies, on chip/board data links, aeronautics, aerospace, sensing, life science diagnostics and environmental monitoring is at an all time high. The availability, performance and cost effectiveness of such photonic components has been inadequate at best. This can partially be attributed to the lack of an efficient, compact and tunable nonlinear element in the optics domain in conjunction with a material system with a viable fabrication technology. III-V semiconductors have been widely used in integrated optoelectronic circuits and nonlinear optical applications. Mature growth, lithography and etching technologies allow the fabrication of low-loss guiding structures. The use of electronic-scale heterostructures enables additional control, flexibility and functionality to be incorporated into the devices. The large useable nonlinearities achievable to date in III-V semiconductors chiefly rely on carriers, and have been studied extensively to induce nonlinearities in semiconductor optical amplifiers. The drawbacks of high insertion loss, large size and excessive noise figures are inherently associated with them and hence they will not be suitable for all applications. Most notable applications that would benefit from integrable ultra-fast optical nonlinearities include monolithically integrated optical parametric oscillators (OPOs), correlated photon pair sources and tunable frequency conversion monolithic arrays.
Optical parametric oscillators (OPOs) have become indispensable coherent sources for the mid infrared. Their operating wavelength span is limited by the transparency window of lithium niobate, however, because periodically poled lithium niobate (PPLN) is the most commonly used nonlinear element in OPOs. In contrast, compound semiconductors such as GaAs exhibit higher nonlinear coefficients near the materials resonances in comparison to PPLN, and have a large transparency window. In the case of GaAs, the transparency window scans the spectral range of 1-17 μm. GaAs also has high damage threshold and a mature fabrication technology for making waveguides in comparison to PPLN. Also, large dispersion is also present in this operation regime. This makes the problems of phase-matching quite severe in this common material system. Various means have been devised to overcome this problem; form birefringence, quasi-phase-matching and photonic bandgap devices were all studied. On the other hand these solutions provided devices that can be difficult to be practically integrated with linear photonic devices, or that involves imperfect phase-matching which reduces the attainable effective nonlinearity.
Correlated photon pairs can be generated through the process of parametric down conversion. For example, one photon at 0.775 μm injected into a sample with appropriate phase-matching could generate two correlated photons at 1.55 μm. Achieving this technology provides very compact sources and allows the integration of the pump source on the same chip. This finds applications in fields ranging from metrology, calibration, quantum experiments, to quantum key distribution.
Ultrafast nonlinearities can also enable the realization of integrated arrays of tunable frequency conversion elements. Optical telecommunications networks which imply any form of wavelength diversity in the physical layer could greatly benefit from these devices. The tuning offered by these devices, together with the integration of the pump source on the same chip, provides unprecedented versatility and configurability into the network. Devices with such functionality have been previously demonstrated in LiNbO3 and have been proven very successful in WDM networks. However, monolithically integrated arrays would vastly reduce system complexity, hence cost.
It is clear from the few applications discussed above that achieving efficient, low loss and tunable phase-matching in a semiconductor material is pivotal for the realization of the next generation of photonic devices. The high nonlinear coefficients of semiconductors at photon energies near the band gap are difficult to exploit practically, as the material dispersion is formidable in this spectral region. This makes phase-matching between the fundamental and second harmonic (SH) waves difficult to achieve. Various authors have proposed schemes to overcome this limitation, utilizing form birefringence as per Reference 1, quasi-phase-matching (QPM) through periodic suppression of the nonlinear coefficient [Reference 1] or domain inversion [References 1 and 2], and resonant cavities [Reference 1]. Such methods generally suffer from difficulty in monolithic integration with other active and passive components, or large insertion loss due to scattering.
What is needed is a relatively simple means for achieving phase-matching in compound semiconductor heterostructures. There is a further need for a device that provides phase-matching in a compound semiconductor structure by means of a waveguide.